1. Field of the Invention
This invention relates to method and apparatus of cementing a casing in a wellbore in which the casing will subsequently be heated to a higher temperature, causing it to want to elongate. It particularly concerns an apparatus whereby the cement can be circulated using conventional optimum methods and then anchoring the lower end of the casing and applying upward tension to the casing until the cement is set.
2. Setting of the Invention
In the search for oil and gas, boreholes are drilled deep into the earth. These holes are lined with casing, which is usually heavy steel pipe, and cement is forced between the casing and the borehole wall. In most cases, the fluid flowing through the wellbore is such that the temperature of the casing doesn't vary much from what it is when it is originally set. However, in a growing number of situations, the fluid flowing through the wellbore is of such a high temperature that the casing is heated to a much higher temperature from that which it was when the casing was set. This is true of thermal wells. A thermal wall can be a well in which steam or other hot fluid is injected down through a tubing string suspended in the wellbore to aid in recovery of fluid from the underground formation, or it can be produced fluid from a formation which has a very high temperature. The increased temperature causes the casing to try to elongate. It has been found that if the casing is hung and cemented and large temperature differences are added to the casing, the tensile stress reduction for the fixed cemented casing is approximately 200 psi per degree Fahrenheit change.
In conventionally cementing a casing string in a wellbore, the casing string is reciprocated and rotated during the placing or circulation of the cement between the outer wall of the casing and the wellbore. The present invention permits conventional placing of cement and then placing the casing under extra tension while the cement sets. The extra-tensile stressing of the casing is thus retained after the cement sets. This extra-tensile stressing prevents heat from causing destructive compressive stresses in the casing once hot fluids are passed therethrough.
3. Prior Art
The closest prior art with which I am familiar is U.S. Pat. No. 3,545,543, Archer W. Kammerer, Jr., and Gary R. Johnson, issued Dec. 8, 1970, for "Casing Apparatus and Method for Tensioning Casing Strings." That patent concerns a method of pre-tensioning the casing string and cementing it in place while its tension condition is retained. However, the method described in that patent has two requirements or disadvantages which the present invention does not have. The method of that patent requires forming a downwardly facing shoulder in the well-bore formation at the lower end of the wellbore; this hole enlargement process is commonly called "under reaming." That patent also teaches to anchor the lower end of the casing and then place the cement between the casing and the borehole wall. This anchoring prior to the placing of the cement prevents the casing from being reciprocated and rotated as is desired in conventional cementing.
The instant invention described herein has a different anchoring system and permits cement to be placed in the conventional manner including the reciprocating and rotating of the casing string prior to anchorage at the lower end of the casing and subsequently applying tension. The present invention also does not require any under reaming for its anchoring.